Operating metal retorts



Aug. 6, 1946. w. M. PEIRCE v 4 OPERATING METAL RETORTS Filed March 10, 1944 INVENTOR W/V/A's A7. Peirce ATTORNEYS Patented Aug. 6, 1946 UNITED STATES ET OFFIC OPERATING METAL an'ronrs Willis M. Peirce, Lehighton, Pa, assignor to The New Jersey Zinc Company, New York, N. Y., a corporation of New Jersey Application March 10, 1944, Serial No. 525,857

3 Claims.

because of compressive stresses, and also to provide a method of operation delaying or inhibiting such collapse. More particularly the invention contemplates an improved method for the repair and maintenance of steel alloy and other metal retorts that have collapsed or tend to collapse in operation. Since the invention is especially applicable to the repair and maintenance of the metal retorts used in the production of magnesium as described in United States Letters Patent of Lloyd Pidgeon, Nos. 2,330,142 and 2,330,143, that application of the invention will be herein particularly described.

Magnesium reduction or smelting is customarily carried out in externally heated metal retorts, usually alloy steel tubes or cylinders-having an internaldiameter of about inches and a wall thiclaness of about to 1 inches. The retorts are now customarily made of a steel alloy containing 25% chromium and nickel. The retorts are mounted in a furnace structure with 5 feet or more of their length in the heated zone. The reducing charge consists of a briquetted mixture of calcined dolomite (or equivalent magnesiferous material) and ferrosilicon (or equivalent reducing agent). The operation is intermittent or batch, and the operating cycle comprises (1) charging the briquets into the retorts, (2) connecting the retorts to a vacuum line capable of maintaining within the retorts a vacuum of 200-250 microns of mercury during the initial stage of the reduction and of 50-100 microns of mercury during the final stage of the reduction, (3) heating the charge to a temperature of 1150- 1250 C. for '7 to 8 hours, (4) removing the condensed magnesium vapor from the cold extension of the retort, and (5) discharging the worked-off briquets or spent residue from the retorts. The metal retorts are thus subjected to atmospheric pressure on the outside and to a relatively highvacuum on the inside during the reducing stage of the operating cycle. This results in a considerable compressive stress on the retort wall. The retort ultimately fails by gradually collapsing under this long continued compressive stress. In the heretofore customary practice, the retorts collapse and must be replaced after an operating period of from 60 to- 90 days.

The present invention is particularly directed towards improving and extending the useful life of the metal retorts used in magnesium reduction or smelting, but is equally well adapted to restoring to its original tubular shape any metal retort or the like that has collapsed (or tends to collapse) on account of exposure to a compressive stress while heated to a high temperature. In accordance with the invention, the highly heated retort (e. g. directly after a residue-discharging operation) is inflated with compressed gas and thus restored to its original shape or form In the case of the metal retorts used in magnesium reduction, the temperature of the retorts will be approximately 1150 C. and compressed air at a pressure of 70 pounds per square inch will restore a collapsed retort to its original shape in a few hours. Thus, the invention involves subjecting such a metal retort, while heated to a temperature approximating its high operating temperature, to an internal gaseous pressure sufficiently high to restore-the retort to substantially its original shape or form.

Where the retort is permitted to collapse before applying the internal gas pressure, a substantial time is required to mund it out, depending on the gas pressure, the retort wall thickness and the retort temperature. Thus, with a 10 inch internal diameter steel alloy retort having a wall thickness of 1 and a retort temperature of 1225 C., the collapsed retort can be completely rounded out in 3 to 4 hours withan air pressure of 80 pounds per square inch. With the same collapsed retort heated to a temperature of 1150 C. and an air pressure of 40 to pounds per square inch, as much as 24 hours is required to round out the retort. After the retort has once been rounded out, it tends (during continued operation) to collapse at intervals of 3 to 6 weeks, depending on the temperature and other conditions, but can be rounded out with compressed air after each collapse. This periodic inflation of collapsed retorts greatly increases the useful life of the retorts.

It is preferable not to wait for the retort to collapse seriously before applying gas pressure to round it out. Thus, it is advantageous to apply internal air pressure of an appropriate amount (say pounds per square inch) for short periods and at frequent intervals, so as to prevent any substantial collapse of the retort. Steel alloy retorts to which an internal air pressure of 70 me ns 3 pounds per square inch is applied for five minutes every second day display a retort life of about three times the average life of the retorts in the heretofore prevailing practice in magnesium reduction.

Compressed air is the most economical and available gas for inflating the retorts, but any appropriate gas may be utilized, especially where air may adversely aifect the highly heated metal of the retort. Thus, inert gases such as nitrogen, carbon dioxide, etc., may be used instead of air.

Any appropriate means may be employed for introducing the compressed gas into the retort,

and for maintaining the contemplated internal gas pressure until the retort has been rounded out or restored to its original form. In the case of the customary retorts used in magnesium reduction, the compressed gas line may be connected to a removable cap adapted to fit over the cold extension of the retort and to be attached thereto, with a gas-tight joint, by toggle bolts, appropriately mounted on the retort or the cap.

The single figure of the accompanying drawing illustrates the application of the invention to a collapsed steel alloy retort 5 in a magnesium reduction furnace of which only the front wall 6 is shown. Some 5 feet or more: of the retort is mounted in the heating laboratory of the furnace, and the compressive stresses to which the originally tubular or cylindrical retort has been subjected has caused its collapse to the shape shown in the drawing. The cold extension I is welded to the retort and accommodates the condenser for the magnesium vapor during the reducing stage of the operating cycle. With the condenser removed, and the worked-oil charge or spent residue withdrawn from the retort, a cap 8 is fitted over the end of the extension 1 and is secured in gas-tight relation thereto by toggle bolts 9. The toggle bolts are pivotally attached to the extension 1 so as to be swung over the customary water jacket 10 into cooperating slots (or holes) in the peripheral flange of the cover. The cover has a gas inlet pipe I l adapted to be coupled to the high pressure gas line l2 for inflating the collapsed retort.

In an actual practice of the invention in a magnesium reduction operation with 10 inch retorts having a wall thickness of 1% inches and operating at a temperature of about 1150 (3., the retorts collapsed after an initial operating period of from to 90 days. After restoring the retorts to roundness by inflation in accordance with the invention, the retorts operated continuously for from 20 to 30 days before again collapsing. From then on, the retorts were rounded out every 20*to 30 days, and the useful life of the retorts was thereby extended almost indefinitely. For example, such retorts have been in continuous operation, with periodic inflation, for over eight months and are still operating satisfactorily. However, as hereinbefore stated, I consider it better practice to avoid marked collapse of the retort by inflation at frequent intervals (say, every second day or so) for a short period of time (say 5 to 15 minutes). However practiced, the invention results in a great extension of the useful operating life of the retorts.

I claim:

1. In the reduction of magnesium in a metal retort subjected while heated for reduction to a high temperature to a compressive stress tending to collapse the retort, the improvement which comprises subjecting the retort while heated to a temperature approximating its high reducing temperature to an internal gaseous pressure of from 40 to pounds per square inch at periodic intervals during the normal operation of the retort, said intervals occurring at times when no charge is in the retort.

2. In the operation of metal retorts in the production of magnesium by reduction where the retorts are subjected during the reducing stage to a high temperature and a compressive stress tending to collapse them, the improvement which comprises inflating the retorts with compressed gas at periodic intervals, said intervals being at times when no charge is in the retorts but when the retorts are heated to a temperature approximating their high operating temperature during reduction and occurring at such frequency as to greatly extend the useful operating life of the retorts.

3. The improvement of claim 2, in which the retorts are inflated with compressed air at a pressure of from 40 to 80 pounds per square inch.

WILLIS M. PEIRCE. 

